The most important trait as a target for crop improvement is yield. Efforts to improve crop yields by developing new plant varieties can be divided into two approaches. One is to reduce crop yield losses by breeding or engineering crop varieties with increased resistance to abiotic stress conditions such as drought, cold, or salt or to biotic stress conditions resulting from pests or disease-causing pathogens. While this approach has value, it does not provide fundamentally improved crop yield in the absence of stress conditions and in fact, such resistance may direct plant resources that otherwise would be available for increased yield in the plant. The second approach is to breed or engineer new crop varieties in which the basic yield capacity is increased.
Classical breeding programs have initially produced substantial gains in improved yield in a variety of crops. A commonly experienced pattern though has been substantial gains in yield initially followed by incremental further improvements that become smaller and more difficult to obtain. More recently developed approaches based on molecular biology technologies have in principle offered the potential to achieve substantial improvement in crop yield by altering the timing, location, or level of expression of plant genes or heterologous genes that play a role in plant growth and/or development. Substantial progress has been made over the past twenty years in identifying plant genes and or heterologous genes that have a role in plant growth and/or development. Despite these gains in using molecular approaches, there continues to be a large unmet need for improved agronomic and horticultural plants produced through more conventional plant breeding. Because of the complexity of plant growth regulation and how it relates in the end to yield traits, it is still not obvious which, if any, of particular genes would be clear candidates to improve crop yield through either plant breeding and/or molecular techniques.
KRP proteins belong to a class of cell cycle inhibitors that bind to and inhibit cyclin/CDK kinase complexes. Mutation of conserved residues within KRP family members are expected to modify KRP's ability to function as an inhibitor of cyclin-CDK kinase complexes. Specifically, some mutations in KRP genes would lead to expression of a nonfunctional KRP cell cycle inhibitor or a cell cycle inhibitor with reduced activity. This loss of or reduced cyclin/CDK kinase inhibitory activity would lead to increased cyclin-CDK kinase activity in cells when normally these cells would have reduced cyclin-CDK activity. This loss of or reduced cyclin/CDK kinase inhibitory activity would lead to increased cell divisions in tissue where the normal wild-type KRP version is expressed. This increased cell division would result in positive agronomic traits such as increased yield, increased weight, size, and/or number of one or more organs, for example, increased seed size, larger plants, larger leaves, larger roots etc. For background on KRP-related technologies, see, for example, WO/2007/016319 and US20070056058, each of which is incorporated by reference in its entirety for all purposes. The present invention identifies new KRP genes and proteins and provides methods for their use in producing improved agronomic and horticultural plants through conventional plant breeding and/or molecular methodologies.